Noble gas and halogen constraints on mineralizing fluids of metamorphic versus surficial origin: Mt Isa, Australia

M A Kendrick, Robert James Duncan, David Phillips

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    The origin of hydrothermal fluids responsible for silica-dolomite alteration associated with Cu ore in the giant Mt Isa deposit of Queensland, north cast Australia has been investigated. Main-stage quartz and dolomite veins, and late- to post-ore quartz from the Buck Quartz Fault in the orebody footwall, plus the regional Mt Isa and Railway Faults contain two phase liquid-vapour fluid inclusions with variable salinity. Fluid inclusion molar Br/Cl values are mostly in the range 1.5-12 x 10(-3) and molar I/Cl values are in the range 9-186 x 10(-6). Greater than seawater Br/Cl and I/Cl values can characterise either sedimentary formation waters with a bittern brine origin by the sub-aerial evaporation of surface water beyond the point of halite saturation or some metamorphic dehydration fluids with a deep crustal source. We propose that these alternative origins can be distinguished in our samples based on the relationship between Ar-40/Ar-36 and Ar-36 concentration. Metamorphic fluids formed by dehydration of crystalline rock are predicted to have low Ar-36 concentrations with the salinity, Ar-40/Ar-36 and Ar-40(E)/Cl values determined by the whole rock concentration of Cl, Ar-36, OH-, K and age. The main-stage quartz and Buck Quartz Fault samples host fluid inclusions with <20 wt. NaCl eq., 1-3 ppb Ar-36, Ar-40/Ar-36 in the range similar to 10,000-29,000 and high Ar-40(E)/Cl values of similar to 3-8 x 10(-4). These values are consistent with metamorphic fluids sourced by devolatilisation of either the underlying Eastern Creek Volcanics or the Barramundi Basement. In contrast, surface-derived fluids acquire elevated Ar-40/Ar-36 values (> 296) through fluid-rock interaction which leads to a simultaneous increase in Ar-36 concentration. The main-stage Ca-rich dolomite fluid inclusions have high salinities close to the point of halite saturation (similar to 26 wt. ), 1.7-5.5 ppb Ar-36, Ar-40/Ar-36 in the range 1600-8400 and Ar-40(E)/Cl of similar to 0.2-1 x 10(-4
    Original languageEnglish
    Pages (from-to)325 - 351
    Number of pages27
    JournalChemical Geology
    Issue number3-4
    Publication statusPublished - 2006

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